The present invention relates to the processing of the signal in spread spectrum digital radiocommunication receivers.
The invention finds application in the field of code-division multiple access (CDMA) radiocommunications, such as UMTS (“Universal Mobile Telecommunication System”).
In a CDMA system, the symbols transmitted, binary (±1) or quaternary (±1±j), are multiplied by spreading codes composed of samples, called “chips”, whose rate (3.84 Mchip/s in the case of UMTS) is greater than that of the symbols transmitted. Orthogonal or quasi-orthogonal spreading codes are allotted to various logical channels sharing the same carrier frequency, so as to allow each receiver to detect the sequence of symbols which is destined therefor, by multiplying the signal received by the corresponding spreading code.
An advantage of the CDMA system is that it makes it possible to utilize radio propagation multipaths, by using what is referred to as a rake receiver.
The rake receiver performs coherent demodulation based on an approximation of the impulse response of the radio propagation channel by a series of spikes, each spike appearing with a delay corresponding to the propagation time along a particular path and having a complex amplitude corresponding to the attenuation and to the phase shift of the signal along this path (instantaneous realization of fading). By analyzing several reception paths, that is to say sampling, several times, the output from a filter matched to the spreading code of the channel, with delays corresponding respectively to these paths, the rake receiver obtains multiple estimates of the symbols transmitted, which are combined to obtain a diversity gain. Combining can be performed in particular according to the so-called MRC (Maximum Ratio Combining) method, which weights the various estimates as a function of the complex amplitudes observed for the various paths.
In order to allow this coherent demodulation, a pilot channel can be provided for the estimation of the impulse response in the form of a succession of spikes. The impulse response is estimated by means of a filter matched to a pilot spreading code with which the sender modulates a sequence of known symbols, for example symbols set to 1. The positions of the maxima of the output from this matched filter give the delays used in the fingers of the rake receiver, and the associated complex amplitudes correspond to the values of these maxima.
The rake receiver comprises a set of fingers making it possible to process one and the same sequence of information symbols that is received along different propagation paths from the sender. These paths are usually defined as delays with respect to a reference time.
In a representation of the CDMA signal received, which representation is utilized in a conventional rake receiver, this signal y(t) may be written:
                              y          ⁡                      (            t            )                          =                                            ∑              k                        ⁢                                          ∑                i                            ⁢                                                                    A                    i                                    ⁡                                      (                    t                    )                                                  ·                                                      b                    k                                    ⁡                                      (                                          t                      -                                              τ                        i                                                              )                                                  ·                                                      c                    k                                    ⁡                                      (                                          t                      -                                              τ                        i                                                              )                                                                                +                      ɛ            ⁡                          (              t              )                                                          (        1        )            where k is an index of the CDMA channels superimposed on the relevant carrier, i is an index of the propagation paths existing between the sender or senders and the receiver, bk(t) is the symbol string shaped by the sender or channel k, ck(t) is the spreading code of channel k, τi is the estimated propagation delay for path i, Ai(t) is the complex amplitude of reception along path i at the instant t, and ε(t) is additive noise.
The receiver selects the paths i of greatest energy, that is to say those for which the Ai(t) have on average the largest modulus. It keeps a predetermined number of paths or the entire set of paths whose energy exceeds a predetermined threshold. The conventional rake receiver supplies each of its fingers, assigned to the reception of channel k along a path i, with the parameters τi and Ai(t) so that this finger can perform:                a signal despreading operation, equivalent to the sampling of the filter matched to the spreading code ck(t) for a delay τi, that is to say:        
                                          Z            i                    ⁡                      (            n            )                          =                              ∑            t                    ⁢                                    y              ⁡                              (                t                )                                      ·                                          c                                  k                  *                                            ⁡                              (                                  t                  -                                      τ                    i                                                  )                                                                        (        2        )            where the sum over t relates to the chips corresponding to the reception of the n-th symbol of the sequence transmitted over channel k. Owing to the orthogonality properties of the codes ck(t), the operation (2) picks out the contribution of path i to the signal, the other contributions being seen as noise after despreading,                then an MRC weighting of the result by the conjugate of the complex amplitude Ai(t), assumed constant (=Ai(n)) over the duration of the n-th symbol, this producing a contribution of path i to the estimate of the n-th symbol:Xi(n)=Zi(n)·Ai*(n)  (3)        
These contributions are ultimately summed at the output of the rake receiver so as to deduce therefrom an estimate X(n) of the n-th symbol:
                              X          ⁡                      (            n            )                          =                              ∑            i                    ⁢                                    X              i                        ⁡                          (              n              )                                                          (        4        )            
In each finger, the despreading operation is that which requires the largest number of elementary operations.
Increasing the number of fingers assigned to a channel, that is to say the number of propagation paths taken into account, improves the reception performance over this channel, by virtue of the multipath diversity. However, this also increases the complexity of the calculations to be performed and hence the use of hardware or software resources in the receiver.
The number of fingers of a rake receiver is generally limited. By way of example, a mobile terminal receiver using CDMA access technology typically possesses from 4 to 8 fingers per channel to be processed.
Good management of the rake seeks to preferably assign these fingers in limited number to the channels which have the greatest need therefor. This makes it possible:                to improve the performance in terms of signal-to-interferers ratio, for a given quantity of calculations; or        to perform a minimum quantity of calculations, for a given performance objective, and hence to make the calculation resources released available for other uses or to reduce the electrical consumption of the receiver (attractive in order to save the battery of a mobile terminal).        
However, such management is not carried out in a satisfactory manner in traditional rake receivers. The fixing of a predetermined number of paths or of fingers leads to mediocre estimates when the channel exhibits greater diversity in terms of paths and to unnecessary calculations when a predominant path or a small number of paths is sufficient to produce reliable estimates. When the paths are taken into account as a function of a comparison of their energy levels with a threshold, the determination of the appropriate threshold is also problematic: one and the same value of threshold may turn out to be both too low in the presence of numerous paths and too high when a powerful path is sufficient to obtain satisfactory estimates.
An object of the present invention is to propose a mechanism which effects a good compromise between the reliability of the estimate of the information carried by the signal received and the calculation cost engendered by the operations of processing the signal in the receiver.